Optimize CPointer.toKStringFromUtf8 on Native

Use strlen and stop allocating intermediate ByteArray
to make the performance comparable to ByteArray.decodeToString().

Partially fix KT-44357.

kotlin-native/Interop/Runtime/src/jvm/kotlin/kotlinx/cinterop/JvmUtils.kt

@@ -21,9 +21,22 @@ import java.io.File
 import java.nio.file.Files
 import java.nio.file.Paths
 
-internal fun decodeFromUtf8(bytes: ByteArray) = String(bytes)
+private fun decodeFromUtf8(bytes: ByteArray) = String(bytes)
 internal fun encodeToUtf8(str: String) = str.toByteArray()
 
+internal fun CPointer<ByteVar>.toKStringFromUtf8Impl(): String {
+    val nativeBytes = this
+
+    var length = 0
+    while (nativeBytes[length] != 0.toByte()) {
+        ++length
+    }
+
+    val bytes = ByteArray(length)
+    nativeMemUtils.getByteArray(nativeBytes.pointed, bytes, length)
+    return decodeFromUtf8(bytes)
+}
+
 fun bitsToFloat(bits: Int): Float = java.lang.Float.intBitsToFloat(bits)
 fun bitsToDouble(bits: Long): Double = java.lang.Double.longBitsToDouble(bits)
 

kotlin-native/Interop/Runtime/src/main/kotlin/kotlinx/cinterop/Utils.kt

@@ -507,18 +507,7 @@ public val String.utf32: CValues<IntVar>
 /**
  * @return the [kotlin.String] decoded from given zero-terminated UTF-8-encoded C string.
  */
-public fun CPointer<ByteVar>.toKStringFromUtf8(): String {
-    val nativeBytes = this
-
-    var length = 0
-    while (nativeBytes[length] != 0.toByte()) {
-        ++length
-    }
-
-    val bytes = ByteArray(length)
-    nativeMemUtils.getByteArray(nativeBytes.pointed, bytes, length)
-    return decodeFromUtf8(bytes)
-}
+public fun CPointer<ByteVar>.toKStringFromUtf8(): String = this.toKStringFromUtf8Impl()
 
 /**
  * @return the [kotlin.String] decoded from given zero-terminated UTF-8-encoded C string.

kotlin-native/Interop/Runtime/src/native/kotlin/kotlinx/cinterop/NativeUtils.kt

@@ -20,9 +20,11 @@ import kotlin.native.internal.Intrinsic
 import kotlin.native.internal.TypedIntrinsic
 import kotlin.native.internal.IntrinsicType
 
-internal fun decodeFromUtf8(bytes: ByteArray): String = bytes.decodeToString()
 internal fun encodeToUtf8(str: String): ByteArray = str.encodeToByteArray()
 
+@SymbolName("Kotlin_CString_toKStringFromUtf8Impl")
+internal external fun CPointer<ByteVar>.toKStringFromUtf8Impl(): String
+
 @TypedIntrinsic(IntrinsicType.INTEROP_BITS_TO_FLOAT)
 external fun bitsToFloat(bits: Int): Float